The invention relates to a method of manufacturing a grid structure with regions exhibiting different properties.
The invention further relates to an X-ray examination apparatus for irradiating an object by means of X-rays, the examination apparatus including an X-ray source, and an X-ray detector, a receiving space for the object to be irradiated arranged between the X-ray source and the X-ray detector, and an X-ray scatter grid with successive regions of different X-ray absorptivity, said X-ray scatter grid to be arranged between the object and the X-ray detector.
Grid structures are commonly used as monitor collimators or as privacy screens. In such cases it is desirable that the grid has a regular structure with, for example regions having different optical properties so that viewing from only one preferred direction can be achieved. On the other hand, it is desirable that the regular structure has a high aspect ratio (with high height-to-width ratio) of the individual regions in the structure.
For the X-ray imaging of objects it is desirable that scattered radiation originating within a patient is attenuated as much as possible by a similar grid structure, commonly referred to as an X-ray scatter grid. The X-ray scatter grid is usually arranged prior to the X-ray detector with respect the to X-ray propagation. When such an X-ray scatter grid is intercepting the beam path behind an object to be examined, for example a patient, the absorption in the patient can be accurately detected in a spatially resolved manner, without scattered radiation from the object being examined leading to uncontrolled intensification of individual areas in the signal measured behind the object.
A method to produce an X-ray scatter grid is known from DE-PS 953 303. The known document discloses a manufacturing method of a supporting member for a scatter grid from a molten mass by way of an extrusion process; said supporting member is provided with regular cut-outs by means of an appropriate comb-like plate. The cut-outs do not extend completely through the supporting member and are subsequently filled with admixtures, notably with metals having a high absorption coefficient for X-rays, thus providing a scatter grid with material strips exhibiting a different X-ray absorption. This process is limited practically by the aspect ratio which can be achieved. Very thin comb-like structures have to be extruded with a very high accuracy. State-of-the-art extrusion is not capable of making structures which are required for present grid designs. The second step, i.e. filling the open channels with an absorbing material requires a low-viscosity material. This limits the percentage of absorbing filler material so that less than one lead equivalent can be achieved. The use of metal strips to insert in the extruded channels is practically not attractive as these have to be fixed in a separate step with very high accuracy. The quality of the X-ray grids is decreased by even small variations in the pitch of the stack due to trimming faults of the material stripes, variations in the viscosity of the glue, dust inclusions and delaminations. Furtheron, with the known method it is not possible to produce a scatter grid with an accurately defined regions of substantial height having a thin cross-section.
It is an object of the invention to produce a scatter grid, where the above mentioned shortcomings are mitigated.
The method according to the invention is characterized in that the material strips exhibiting different properties are extruded so as to form the regions of said grid structure. By applying a per se known extrusion process to manufacture the grid structure, a one-step process is achieved, where all material layers are combined, leading to no limitations in the height of the layered structure. Secondly, because the successive regions of different properties are formed by extruded material strips in accordance with the invention, they can be manufactured in a similar manner. Each time layers that are closed in themselves are formed and arranged one on top of the other, thus forming a clean, well-defined interface with one another. The method can be performed continuously so that the execution is faster and with reduced manufacturing costs. Thus it is a process which can be scaled up easily and the manufacturing costs do not increase with decreasing pitch and hardly with increasing size. Material interfaces are generated in situ and are not exposed to air, reducing probability of contamination.
The manufacture is significantly simplified notably when two material strips of different properties are co-extruded; moreover, the layers thus formed are cooled simultaneously. The bonding of the layers to one another is thus optimized while at the same time a well-defined interface is formed.
An embodiment of the method according to the invention is characterized in that the grid structure is an X-ray scatter grid with successive regions having different X-ray absorption coefficients, characterized in that material strips exhibiting a different X-ray absorption behavior are used. Such an assembly, notably comprising a plurality of strips in an alternating arrangement, can be deformed as a unit so as to realize a thickness in the millimeter range in the direction of the beam path. It is also advantageously possible to realize focusing by realizing an inclination of individual strips, for example by means of an extrusion die, and by subsequently performing a deformation during which the inclination of the strips is obtained.
The multiplication of the alternating succession of strips is effectively realized by means of a device for the multiplication of material strips which divides the material strips in the direction perpendicular to the longitudinal direction of the co-extruded material strips and compresses said material strips so as to form a packet of reduced height, after which stacking takes place. In a typical implementation the number of strips is thus doubled in each multiplication step, but also multitudes of this operation can be realized in a single element by dividing the melt stream in a multitudes of channels.
Further advantages and details of the invention will become apparent from the description of the following embodiment of the invention which is illustrated in the drawings.